Research on transparent conducting oxides (TCOs) that offer suitable alternatives to indium tin oxide (ITO) has attracted considerable attention. This is due to the serious concern relating to the cost of ITO, as well as chemical stability in a reduced ambient. The cost of ITO is attributable to the high cost of indium (In) and its limited availability, which is due in part to the rapid increase in production of flat panel displays utilizing ITO. ITO is utilized in numerous types of optoelectronic devices. In particular, organic photovoltaic (OPV) cells have attracted considerable attention in recent years due to their potential for providing low cost solar energy conversion. Since the first bilayer heterojunction solar cell was demonstrated in 1986, there has been considerable development in the field of OPVs. It has recently been demonstrated that solar cells with 3.6±0.2% efficiency can be fabricated using double heterojunction on (ITO) substrates. However, the use of ITO poses a serious problem for the reasons noted above.
Zinc oxide (ZnO) doped with Group III elements (typically aluminum or gallium) is a promising candidate as a TCO because of its superior stability in a hydrogen environment, benign nature and relatively inexpensive supply. These materials have shown promising results when used as anodes in organic photovoltaic (OPV) cells (e.g., organic solar cells (OSCs)), organic light emitting diodes (OLEDs) and other optoelectronic devices.
Highly conducting and transparent Ga:ZnO can be deposited on single-crystalline sapphire substrates as well as glass substrates using pulsed laser deposition (PLD). See Bhosle et al., Appl. Phys. Lett. 88, 32106 (2006); Bhosle et al., Appl. Phys. 100, 033713 (2006). The electrical properties of the TCO films are determined by the details of microstructure, stoichiometry and defects, which in turn can be controlled by the processing and substrate parameters. The formation of textured <0001> films on amorphous glass substrates poses a major technical challenge in terms of the ability to control the grain size in the nanometer range, the texture, and the grain boundary characteristics in these films to achieve superior properties and device (e.g., solar cell) performance.
The suitability of these nanocrystalline ZnGa0.05O films deposited on glass as the anode of a double heterojunction OPV cell has been demonstrated with power conversion efficiencies >1%. See Bhosle et al., J. Appl. Phys. 102, 023501 (2007). The power conversion efficiencies of the ZnGa0.05O based cell were comparable to an ITO-based OPV cell, even though the ZnGa0.05O based cell showed relatively higher contact resistance. This was attributed to the film surface and interface characteristics of ZnGa0.05O with the organic layer owing to the lower work function of ZnO-based TCOs, which determines the energy level alignment at the ZnGa0.05O/organic layer interface.
In the present disclosure, it is proposed that while low resistivity and high % T (transmittance) are prerequisites for superior device performance, another important parameter is the surface work function of the TCO. The lower work function of conventional TCOs makes it difficult to achieve ohmic contacts at the interface, which increases the series resistance and limits the realization of maximum theoretical open-circuit voltage (Voc) in OSCs. In the case of OLEDs, the surface work function affects the energy barrier height at the interface of the TCO with the organic semiconductor layer, playing a role in enhancing the hole injection efficiency and reducing the operating voltages of the device. Another important issue is the interface stability between the TCO and the organic layer in the device, which is critical for reliability and long-term performance. Therefore, the diffusion barrier characteristics of the TCO play an important role in this regard.
Accordingly, there is a need for non-ITO based TCOs, and methods for fabricating such TCOs, which exhibit high work function, high optical transparency, low sheet resistance, high conductivity (low resistivity), and excellent interface stability, particularly as compared to ITO-based TCOs. There is also a need to for non-ITO based TCOs that can be made commercially available at lower cost as compared to ITO-based TCOs.